1. Field of the Invention
The present invention relates generally to a digital tuning receiver, and is directed more particularly to an AFT circuit thereof.
2. Description of the Prior Art
A prior art television receiver which uses a digital tuning system is described with reference to FIG. 1. An electronic tuner 1 is electronically tuneable using, for example, a variable capacitance diode (not shown) as a resonant element in its respective tuning circuit. The electronic tuner 1 is tuned by a tuning voltage V.sub.c generated by a tuning control circuit shown generally at 10. The tuning control circuit 10 has two modes of operation. In a first mode, tuning is swept smoothly upward or downward under the control of up-sweep switch S.sub.u and down-sweep switch S.sub.d. When the appropriate switch is operated, pulses from pulse oscillator 14 are counted up or counted down in a reversible counter 13. The digital number stored in reversible counter 13 is D to A converted in D/A converter 18 to generate the channel selecting voltage V.sub.c. When a channel is located by the sweep method, the sweep is automatically stopped and the digital number in reversible counter 13 may optionally be stored in a channel memory 11.
In the second mode of operation, tuning control circuit 10 selects channels previously stored in channel memory 10 under control of channel switches S.sub.1 -S.sub.10. In this case, the digital number previously stored in channel memory 11 for a given channel is read out, D to A converted in D/A converter 18 and thus directly generates channel selecting voltage V.sub.c to tune tuner 1 to the selected channel.
In each mode of operation, an automatic fine tuning device compensates for minor misalignment of the tuner by correcting the digital number employed in generation of channel selecting voltage V.sub.c. A conventional video intermediate frequency (VIF) amplifier 2 is connected to electronic tuner 1, a video detector 3 is connected to the VIF amplifier 2, a video amplifier 4 is connected to the video detector 4, and the output signal from the video amplifier 4 is applied to a television picture tube 5. Upon channel selection, the digital channel data A.sub.1 -A.sub.n from channel memory 11 is connected to D/A converter 18 which produces the channel selecting voltage V.sub.c corresponding to the desired broadcast channel. The channel memory 11 contains addresses whose number corresponds to TV channels to be received, for example, TV channel 10. Memory 11 is preferably non-volatile in order to retain the contents therein even in the event that power is lost. An addressing circuit 12 is provided to which channel selecting switches S.sub.1 to S.sub.10 are connected. When one of the switches S.sub.1 to S.sub.10 is operated, a corresponding channel address in memory 11 is selected and connected to D/A converter 18. A reversible counter 13 is employed to initially generate the codes A.sub.1 to A.sub.n for the sweep mode and for channel selection. A pulse oscillator or generator 14 supplies pulses P.sub.o to reversible counter 13. A memory control circuit 15 controls the storage of addresses from reversible counter 13 in channel memory 11. A mode switch S.sub.m determines whether tuner 1 is scanned looking for a signal or whether it is controlled by channel memory 11. By placing mode switch S.sub.m in the R position, the apparatus is placed in a write-in (preset) mode in which scanning is performed until a signal having a predetermined characteristic is found, and the scanning is then stopped and the digital value representing the channel is stored in channel memory 11. When switch S.sub.m is placed in the P position, the apparatus is placed in a read-out (tuning) mode in which tuning is controlled by channel memory 11 15.
A write pulse forming circuit 16 generates a pulse which enables memory control circuit 15 to store the number from reversible counter 13 in channel memory 11. A reset circuit 17 generates a reset pulse under control of mode switch S.sub.m to erase the number in reversible counter 13.
In FIG. 1, there are also provided circuits 21 to 23 which receive the output signal from the VIF amplifier 2 and produce the VIF signal from the VIF amplifier 2 is applied through a band pass amplifier 21 of AFT circuit 70 to a frequency discriminator 22 which then produces a typical S-shaped AFT voltage V.sub.s as shown in FIG. 2A. This voltage V.sub.s is fed to a level detecting circuit 23 which has threshold levels V.sub.1 and V.sub.2 and produces the AFT signals V.sub.u and V.sub.d as shown in, for example, FIG. 2B when the S-shaped voltage V.sub.s exceeds the positive threshold V.sub.1 or the negative threshold V.sub.2. AFT signal V.sub.u becomes 1 in the frequency range between (f.sub.o -1.2 MH.sub.z) and (f.sub.o -50 KH.sub.z) where f.sub.o represents the normal tuning point or frequency and becomes 0 in the frequency ranges other than the above frequency range, while the AFT signal V.sub.d becomes 1 in the frequency range between (f.sub.o +50 KH.sub.z) and (f.sub.o +1.2 KH.sub.z) and becomes 0 in the other frequency ranges. In this case, the frequency range (f.sub.o .+-.50 KH.sub.z) become a tuning range.
In FIG. 1, a logic circuit formed of circuit elements 31 to 39 controls reversible counter 13 and write pulse forming circuit 16. When the switch S.sub.u or S.sub.d is placed in the ON position, a train of pulses P.sub.o from pulse oscillator 14 is supplied to the up or down input of counter 13.
Accordingly, when television channels are to be located by sweeping, the mode switch S.sub.m is placed in the P position with its movable element in contact with its preset contact P. Counter 13 is reset by a reset pulse generated by reset circuit 17 due to switching mode switch S.sub.m into the P position. Channel memory 11 is placed in the write-in mode by the memory control circuit 15.
Thus, when switch S.sub.1 and up-sweep switch S.sub.u are both operated, the address corresponding to the switch S.sub.1 in channel memory 11 is addressed through addressing circuit 12 to receive and store the digital number generated in reversible counter 13. The output from the inverter 31 becomes 1 upon operation of switch S.sub.u. This enables one input of AND gate 32. Pulses P.sub.o from pulse oscillator 14 are supplied through AND gate 32 and OR gate 33 to the UP input of reversible counter 13. Thus, the number is reversible counter 13 increases sequentially from zero [00---0]. The digital number in reversible counter 13, fed to the D-A converter 18, produces the channel selecting voltage V.sub.c corresponding to the bits A.sub.1 to A.sub.n of reversible counter 13. This voltage V.sub.c increases by .DELTA.V each time the content of the counter 13 increases by 1. As a result, the frequency to which the tuner 1 is tuned increases sequentially to achieve an upward sweep of the received frequency.
If the above sweep operation is performed in the Tokyo area which has a TV channel, for example, when the station broadcasting channel 1 is received, up-sweep switch S.sub.u is released. Thus, the tuning for the desired channel can be achieved. When the up-sweep switch S.sub.u is released slightly late and hence the received frequency is higher than the desired frequency, down-sweep switch S.sub.d may be operated. Down-sweep switch S.sub.d causes inverter 34 to generate a 1 which enables one input of AND gate 34. A train of pulses P.sub.o from pulse oscillator 14 is fed through the AND gate 35 and the OR gate 36 to the down input of reversible counter 13. Thus, channel selecting voltage V.sub.c is swept lower to achieve fine tuning of the received frequency.
When the switch S.sub.u (or S.sub.d) is released, the output from AND gate 37 becomes 1. This enables a first input of AND gates 38 and 39. If the received frequency is a little lower than the desired frequency, the AFT signal V.sub.u is 1 at this time. AFT signal V.sub.u enables a second input of AND gate 39. With two of its inputs enabled, AND gate 39 connects a train of pulses P.sub.o from pulse oscillator 14 through OR gate 33 to the up input of reversible counter 13. This causes the tuning frequency to increase. When the tuning is moved upward to within 50 KH.sub.z of channel 1, the AFT output becomes 0. This inhibits AND gate 39 and halts further increase in the number in reversible counter 13. Similarly, if the received frequency is a little higher than the desired frequency, the AFT signal V.sub.d is 1, the trains of pulses P.sub.o from pulse oscillator 14 is delivered through AND gate 39 and OR gate 36 to the down input of reversible counter 13 when V.sub.u =V.sub.d =0, the received frequency is within .+-.50 KH.sub.z of the precise frequency f.sub.o. A frequency deviation of no more than 50 KH.sub.z is considered correct tuning and has no substantial effect on the received picture.
Write pulse forming circuit 16 is enabled by the 1 from AND gate 37 at the end of operation of up-sweep switch S.sub.u or down-sweep switch S.sub.d. Write pulse forming circuit 16 applies a delay long enough to permit the AFT circuit 70 to complete its fine tuning function and then produces a write pulse. The write pulse from the write pulse forming circuit 16 is fed through the memory control circuit 15 to the channel memory 11 to erase the content thereof at the address designated by switch S.sub.1 and to write the contents of reversible counter 13 into that address. Accordingly, the part of the channel memory 11 corresponding to the address designated by the switch S.sub.1 contains channel selecting codes A.sub.1 to A.sub.n corresponding to the channel selecting voltage V.sub.c for channel 1.
Next, switches S.sub.2 and S.sub.u may be operated. The address of memory 11 corresponding to the switch S.sub.2 is designated for change and the content of the counter 13 reversible is further increased from the value stored in it from the preceding operation. The selecting voltage V.sub.c continues to sweep the received frequency higher until the next broadcast station above channel 1, for example, channel 3 in the Tokyo area is received. Up-sweep switch S.sub.u is released, a delay is imposed by write pulse forming circuit 16 to permit completion of the AFT operation, then digital number A.sub.1 -A.sub.n in reversible counter 13, corresponding to channel 3 is written into channel memory 11 at the address designated by switch S.sub.2. Similarly, the remaining television channels for a given area can be preset in memory 11.
When mode switch S.sub.m is placed in the R position, and one of channel selecting switches S.sub.1 to S.sub.10 corresponding to a desired channel is operated. Channel memory 11 is place in its read-out mode by memory control circuit 15. At this time, if switch S.sub.2, for example, operated, the number stored in the corresponding address channel memory 11 corresponding to channel 3 is read out. The read-out codes A.sub.1 to A.sub.n are fed through the counter 13 to the D-A converter 18 to generate the channel selecting voltage V.sub.c which is then fed to the tuner 1. Thus, the channel 3 can be received.
Further, in this case if the received frequency is by mistuned by more than 50 KH.sub.z due to temperature variation in electronic tuner an AFT voltage V.sub.u or V.sub.d is generated as previously described. An appropriate amount is added to or subtracted from the number in reversible counter 13 to correct the selecting codes A.sub.1 to A.sub.n, which are fed from the counter 13 to the D-A converter 18 until the AFT voltage is removed.
According to the prior art tuning control circuit 10, however, upon sweeping the received frequency in the presence of a strong signal which is not subjected to automatic gain control (AGC), false AFT signals are generated.
The frequency response of the VIF amplifier 2 is as shown in FIG. 3A. The pass band of band pass amplifier is shown between dot-dash lines 72a and 72b in FIGS. 3B-3I. The S-shaped voltage V.sub.s which is generated when the picture carrier signal F.sub.p1, for example, of channel 1 is swept past the band pass amplifier 21 is shown in FIG. 3B. In the figures, f.sub.s and f.sub.p represent a normal sound intermediate frequency and a normal video intermediate frequency, respectively.
For the sake of simplicity it is assumed that for the above characteristic an AGC voltage V.sub.a for the tuner 1 is such that the AGC voltage V.sub.a is 1 only when the sound carrier frequency f.sub.s signal or video carrier frequency f.sub.r in a received signal are in the frequency range of VIF amplifier 2 between 54.25 MH.sub.z and 60.25 MH.sub.z and is 0 in all other frequency bands as shown in FIG. 3C. accordingly, when V.sub.a =1, the AGC operation is carried out and hence the gain of the tuner 1 is reduced, while when V.sub.a =0 no AGC operation is carried out and hence the tuner 1 is at full gain.
FIGS. 3D-3I show a sequence of conditions in which the signals and beat signals of two adjacent channels are swept in the rightward direction past the stationary frequency response curves of VIF amplifier 2 and band pass amplifier 21. For normal reception, as shown in FIG. 3D, sound and video intermediate frequency signals F.sub.s1 and F.sub.p1 in the desired channel are positioned at the frequencies f.sub.s and f.sub.p. Further, in normal television broadcast, band allocations, only alternate channels are occupied in a given area. This provides a guard band of 6 MH.sub.z between adjacent allocated channels. Thus the nearest video intermediate frequency signals F.sub.s2 and F.sub.p2 of an adjacent station are positioned 12 MH.sub.z from signals F.sub.s1 and F.sub.p1.
A strong signal containing F.sub.s1 and F.sub.p1 can generate beat signals F.sub.u1 and F.sub.d1 a television receiver due to mutual modulation of the carrier signals. Beat signal F.sub.u1 is positioned 4.5 MH.sub.z above F.sub.s1. Beat signal F.sub.d1 is positioned 4.5 MH.sub.z below F.sub.p1. Similarly, quasi-signals f.sub.u2 and F.sub.d2 are generated by mutual modulation of signals F.sub.s2 and F.sub.p2, respectively and are positioned 4.5 MH.sub.z above and below them respectively.
When a television is properly tuned as shown in FIG. 3D, V.sub.a =1 and the AGC is applied to the electronic tuner 1 and hence its gain is low. F.sub.u1 to F.sub.d2 are attenuated to such a low level that they are essentially not a factor. This is illustrated in FIGS. 3D and 3G-3I by showing these signals as broken arrows.
When tuning has progressed to the condition shown in FIG. 3E, signals F.sub.s1 and F.sub.p1 are moved outside the passband of VIF amplifier 2 shown in FIG. 3A but adjacent signals F.sub.s2 and F.sub.p2 have not yet been moved inside the passband. Since there is neither a picture nor a sound carrier within the VIF passband, the AGC voltage is low. Thus, no AGC is applied to the electronic tuner 1 and hence electronic tuner 1 and other circuits are at full gain with the result that the quasi-signals F.sub.d1 and F.sub.u2 are generated within the VIF passband and also signal F.sub.u2 is within the bandpass of band pass amplifier 21. Note that signal F.sub.u2 is between band edge 72b and cross-over point 72c. Thus, a positive AFT voltage V.sub.s is generated by F.sub.u2 in this location. This time is indicated at time t.sub.10 in FIGS. 4A-4K.
A short time later, denoted as t.sub.11 in FIG. 3F and FIGS. 4A-4K, tuning has progressed to the point that F.sub.u2 is approaching crossover line 72c (FIG. 3F). But note that, at this same time, the picture carrier signal F.sub.p2 is just outside the edge of the VIF passband. An instant later, at time t.sub.12 beat signal F.sub.u2 is located between crossover line 72c and band edge 72a. This position of beat signal F.sub.u2 would produce a negative voltage V.sub.s (FIG. 3B) except for the fact that, just as signal F.sub.u2 crossed crossover line 72c, picture carrier f.sub.p2 entered the VIF passband. The presence of F.sub.p2 in the VIF passband reestablishes a strong AGC signal. Thus signal F.sub.u2 is suppressed, as shown by the broken arrow, to such an extent that it is incapable of generating a negative alternation of V.sub.s.
Still later, at time t.sub.13, beat signal F.sub.u2 is outside the VIF pass band and F.sub.d1 is located between crossover line 72c and 72a. However, as before, the presence of the picture carrier F.sub.p2 within the VIF passband suppresses beat signal F.sub.d1 and prevents generation of a negative component of V.sub.s.
With continued tuning, picture carrier signal F.sub.p2 enters the passband of the band pass amplifier 21 between band edges 72b and 72a. At time t.sub.14 (FIGS. 3I and 4A-4K), the picture carrier signal F.sub.p2 is aligned with crossover line 72c. Unless disturbed, the AFT system will maintain this condition.
As a result, when the received frequency is swept from left to right, as shown in FIGS. 4A to 4C, a false S-shaped voltage V.sub.u having only a positive alternation corresponding to FIGS. 3F and 3E is generated between times t.sub.2 and t.sub.3, and then the normal positive and negative S-shaped voltage V.sub.s and AFT signals V.sub.u and V.sub.d are obtained in correspondence with FIGS. 3I between times t.sub.5 and t.sub.8.
If the direction of frequency sweep is reversed, the sequence of positions shown in FIGS. 3I to 3D produce the same S-shaped voltage V.sub.s and AFT signals V.sub.u and V.sub.d as previously described but in the reverse order as shown in FIGS. 5A to 5C.
When a false signal V.sub.u, such as at times t.sub.10 -t.sub.11, other than the normal AFT signals V.sub.u and V.sub.d is generated as set forth above, the prior art tuning control circuit 10 stops the sweep operation the frequency position at which the false AFT signal V.sub.u becomes "0". Accordingly, receiver tuning stops on a false signal, such as F.sub.u2. This mis-tuning gives a picture in which proper synchronization cannot be achieved, the higher frequency components are missing and no color appears. The resulting picture quality is seriously deteriorated with the normal picture screen.